This invention relates to a method and apparatus for the conversion of coal to gas, liquid and solid products. More particularly, the application relates to a method and apparatus in which coal is converted to gas, liquid and solid products by a integral combination of pyrolysis, gasification and possibly Fischer-Tropsch synthesis.
Methods and apparatus for the conversion of coal to gas, liquid and solid products (for example coal gas, coal tar and coke respectively) have been known for a long time. In the classic method for the coking of coal, coal is subjected to pyrolysis in air-tight retorts. A gaseous product is evolved which, upon cooling, yields liquids comprising coal tar and other organic chemicals, and coal gas. The solid residue left in the retort after the pyrolysis is coke. This conventional pyrolysis procedure has a number of disadvantages; the gas evolved is relatively high in carbon dioxide and low in hydrocarbons since most of the organic hydrogen in the coal ends up in the liquid stream or as molecular hydrogen in the gas, while the solid residue tends to cake and adhere to the walls of the retort. The proportion of hydrocarbon in the gas product can be improved somewhat by subjecting the gas stream to a Fischer-Tropsch reaction (the reaction of carbon monoxide and hydrogen over a catalyst, usually a metal oxide catalyst, to produce hydrocarbons and carbon dioxide), but the caking problem is sufficiently severe to prevent certain types of coal being used in such a pyrolysis reaction. In addition, such pyrolysis reactions are very energy-intensive because the coal must be maintained at a high temperature for several hours.
It is also known to subject coal, other organic materials and char derived from either coal or other organic materials to a so-called gasification reaction, in which the coal or the like is reacted with oxygen and steam to produce a gas containing hydrogen and carbon monoxide. This gasification reaction can be made to generate an amount of heat.
We have now discovered that the pyrolysis of coal can be carried out to a larger extent by conducting the pyrolysis in the presence of gas generated by subjecting the solid residue of the pyrolysis reaction to a gasification reaction.